This invention relates to an apparatus for providing uniform contact of a downwardly moving, compact bed of particulate solids with a liquid which floods the bed. More particularly, the invention is concerned with an apparatus having means for insuring relatively uniform, gravity flow, often referred to as plug flow, of a compact bed of particulate solids through a vessel containing the solids in contact with a liquid which floods the bed to fill the interstices between the solids over a substantial height of the vessel. The apparatus is especially useful in contacting the solids with a liquid treating agent in order that the solids passing through the vessel have a relatively uniform residence time in contact with the liquid thereby insuring that given portions of the solids will not be treated for materially greater or lesser times than desired. As an example, the treatment of corn solids with an aqueous solution of calcium hyroxide can be accomplished in a continuous manner in the apparatus of the invention at relatively uniform residence times. Such treatment facilitates the further processing of the corn without having to unduly adjust subsequent treating operations as might be required if the products withdrawn from the treating vessel were not subjected to relatively uniform treatment with the liquid.
In many processes it is desired to contact particulate solids with liquids for a given period of time in order to accomplish a desired chemical or physical modification of the solids. When large amounts of solids are to be treated, it is most advantageous that they be disposed as a relatively compact bed in order to contain the solids in the smallest possible vessel and thereby save the considerable expense that the use of larger vessels would entail. In the compact bed it is desirable that there be little, if any, relative movement of the particles with respect to each other and, in the type of treatment involved, there is no need to provide agitation or intimate mixing of the liquid and solids since mere flooding of the bed with the treating liquid will suffice to accomplish the desired result.
These treatments of solids have most often been accomplished in the past merely by providing a number of soaking tanks in which the solids are placed and then flooded with the liquid treating agent. After a period of time, the tanks are emptied and the operation repeated. This type of batch operation requires a plurality of treating tanks, allotment of periods for loading and unloading, and excessive labor cost. It is more desirable to conduct the operation on a more continuous basis to reduce the vessel capacity required and lower operating expense.
One approach to a more continuous type of treatment is to feed the solids into the top of a treating vessel containing the treating liquid and remove treated solids from the bottom, and, in doing so, it is desired that the solids be disposed as a compact, downwardly moving bed in order to utilize a vessel of relatively small capacity for the amount of solids to be processed. Although appearing quite simple, difficulties in this operation do arise. In order to obtain a uniform length of contact between the solids and the treating liquid, any given solid particle should have approximately equal residence time in the liquid. To accomplish this goal, the compact bed should move relatively uniformly, downwardly through the vessel across substantially the entire cross-section of the vessel. This type of flow has often been characterized as plug flow and indicates the substantial absence of flow channels through the bed wherein some particles move faster than in other portions of the bed.
The apparatus of the present invention provides for relatively uniform, downward movement by gravity of the compact bed of particulate solids through a treating vessel while in contact with a treating liquid which occupies or floods the vessel over a substantial portion of its height. The flooding liquid thus occupies the interstices between the solid particles which are in particle-to-particle contact substantially throughout the compact bed. Also, the movement of the bed through the vessel is sufficiently slow so that there is relatively little particle-to-particle movement or intermixing, and substantial channeling of particle flow through the bed is avoided. This operation is made possible by providing the vessel with especially designed means for controlling the flow of solids through or from the bed in the region of its lower portion. The vessel may also be provided with means to distribute the solids onto the top of the bed.
The invention is particularly useful in treating grains with a liquid which serves to facilitate dehulling. For example, solids such as whole corn grains can be contacted with an aqueous solution of lime at, for instance, temperatures of about 95.degree. F. to 145.degree. F. Suitable treating times include, for instance, about 6 to 16 hours or more.
The character and operation of the features of the present invention will become apparent from the description of embodiments of the invention as represented in the drawings in which like parts are designated by like reference numerals.